1. Field of the Invention
This invention relates to a non-linear deemphasis circuit which is suitably applied, for example, to a video tape recorder.
2. Description of the Prior Art
Conventionally, video tape recorders are designed such that a non-linear emphasis circuit and a non-linear de-emphasis circuit are employed in order to increase the signal-to-noise (SN) ratio of a reproduction signal.
An exemplary conventional non-linear emphasis circuit is shown in FIG. 7. Referring to FIG. 7, the non-linear emphasis circuit shown is generally denoted at 1 and is constituted such that high frequency components of a recording signal S.sub.REC are extracted by a high-pass filter circuit (HPF) 2 and then the recording signal S.sub.REC is outputted to an adding circuit 4 by way of a limiter circuit 3.
High frequency components of the recording signal S.sub.REC are amplitude limited by the limiter circuit 3 as seen from a curve L1 shown in FIG. 8.
The output signal of the limiter circuit 3 is added to the recording signal S.sub.REC by the adding circuit 4 to produce an output signal S.sub.OUT to be recorded on a magnetic tape. Consequently, the recording signal S.sub.REC is recorded with its high frequency components of small amplitudes emphasized as seen from another curve L2 shown in FIG. 8.
A conventional non-linear de-emphasis circuit is shown in FIG. 9. Referring to FIG. 9, the non-linear de-emphasis circuit is generally denoted at 5 and is constituted such that an input reproduction signal S.sub.RF is applied to a differential amplifier circuit 6, and a feedback loop to the differential amplifier circuit 6 is formed by a high-pass filter circuit 7, a limiter circuit 8 and an adding circuit 9.
Characteristics of the high-pass filter circuit 7, limiter circuit 8 and adding circuit 9 are set to the same characteristics (shown by a curve L3 in FIG. 10) as those of the high-pass filter circuit 2, limiter circuit 3 and adding circuit 4 of the non-linear emphasis circuit 1 so that a reproduction signal S.sub.RF may be corrected with an inverse characteristic to that of the non-linear emphasis circuit 1. The output signal of the non-linear de-emphasis circuit 5 is illustrated by a curve S.sub.RF1 shown in FIG. 10.
Consequently, the transfer function of the recording and reproducing system is 1 and noise in small amplitudes in high frequency components is suppressed.
Another exemplary conventional non-linear de-emphasis circuit is disclosed in Japanese Patent Laid-Open Application No. 62-123881 and shown in FIG. 11. Referring to FIG. 11, the non-linear deemphasis circuit is generally denoted at 10 and is constituted such that a limiter circuit 12 is provided separately to effect amplitude limiting in order to further raise the improvement in SN ratio.
In particular, an input reproduction signal S.sub.RF is received at a subtracting circuit 13, and an output signal of the subtracting circuit 13 is fed back to the subtracting circuit 13 by way of a high-pass filter circuit 7 and a limiter circuit 8.
Thus, at the subtracting circuit 13, an output signal of the limiter circuit 8 is subtracted from the input reproduction signal S.sub.RF. Accordingly, a similar characteristic to that of the non-linear de-emphasis circuit 5, wherein a feedback loop to the differential amplifier circuit 6 is formed as described hereinabove with reference to FIG. 9, is obtained.
Subtraction of the output signal of the limiter circuit 8 from the reproduction signal S.sub.RF is performed, in the non-linear de-emphasis circuit 10, by a subtracting circuit 16. The output signal of the limiter circuit 8 is transmitted to the subtracting circuit 16 by way of the limiter circuit 12 and an attenuator 14.
As seen from FIG. 12, the limiter circuit 12 further amplitude limits its input signal at a second amplitude limiting value LIM2 which is lower than a maximum output amplitude, i.e., the first amplitude limiting value LIM1 of the limiter circuit 8.
The gain x of the limiter circuit 12 and attenuator 14, when the amplitude of the output signal of the limiter circuit 8 is small, is maintained so that it satisfies the following expression: ##EQU1## where A is a gain of the high-pass filter circuit 7 and limiter circuit 8 for a small amplitude of a high frequency and .alpha. is a gain of an attenuator 18 interposed between the limiter circuit 8 and the subtracting circuit 13. Thus, the input/output characteristic of the limiter circuit 12 and the attenuator 14 is as illustrated by a curve L5 shown in FIG. 12.
Consequently, an input/output characteristic is obtained wherein high frequency small amplitudes are suppressed, compared with the non-linear de-emphasis circuit 5, in a region a over which the limiter circuit 12 does not perform amplitude limitation as seen from FIG. 13.
Accordingly, high frequency small amplitudes are suppressed by the entire recording and reproducing system, as seen from a curve L7 shown in FIG. 14, and the SN ratio improvement achieved using the non-linear de-emphasis circuit 10 is increased with respect to the SN ratio improvement achieved using the non-linear de-emphasis circuit 5.
However, while the improvement in SN ratio of the non-linear de-emphasis circuit 10 is high, the high frequency small amplitudes are suppressed in the region a also include desired input signal components and as a result, there is a problem in that a delicate contour of a reproduction picture image is damaged since input signal high frequency small amplitude components are undesirably suppressed.